Certain test and measurement systems incorporate a switching mechanism that allows any one of perhaps several pieces of test equipment to be connected to any one of perhaps several devices under test. Commonly encountered switching topologies include matrices and multiplexers. The test equipment may include sources such as power supplies and oscillators, and measurement devices such as voltmeters and analyzers. It is common in such systems to use coaxial cable to interconnect the test equipment and the devices under test. Many types of measurements require that both the center conductor and the shield of the coaxial interconnection be disconnected when a switch is opened, just as if an actual coaxial connector were being removed. Examples include floating measurements and ground loop elimination.
It is not common for a coaxial relay to switch both the center conductor and the shield; most switch only the center conductor. A low cost way to approximate the action of a coaxial connector with conventional coaxial relays would be desirable. This may be accomplished through the use of two coaxial relays and a third relay. Each coaxial relay is connected to one of the coaxial cables to be switched: the center conductor of each coaxial cable is connected to the center conductor of its associated coaxial relay and the shield of each coaxial cable is connected to the shield of its associated coaxial relay. The remaining center conductor terminals of the coaxial relays are connected together directly, but the shields of the two coaxial relays are connected through the third relay. All three relays open and close together. Thus the coaxial switching mechanisms are each fully shielded, whether the coaxially relays are opened or closed, yet the coaxial shields are separated when the coaxial relays are opened.
The three-relay configuration described above achieves excellent isolation when open, but is unfortunately susceptible to certain crosstalk mechanisms between pairs of closed such three-relay configurations. The crosstalk may be adequately suppressed by the use of properly placed ferrite cores to form common mode chokes.